The present invention relates to dry vacuum pump units for establishing a fine vacuum, for use in particular in the semiconductor industry to lower the pressure in process chambers, down from atmospheric pressure.
The present invention relates more particularly to dual-rotor vacuum pump units comprising a pump stator with at least one axial internal cavity receiving two parallel pump rotors mounted to rotate on corresponding bearings and coupled at a first end via gears enclosed in a casing containing oil. The first end of one of the rotors is extended by a coaxial drive shaft which is engaged in the rotor of a motor for driving the vacuum pump. The motor has a stator with a stator coil and is enclosed in a motor casing running on from the casing containing the gears and oil. Known structures for vacuum pump units are described in EP 0 733 804, U.S. Pat. Nos. 5,904,473, 2,940,661, and JP 60 259791.
In vacuum pump units, one of the difficulties is to provide satisfactory sealing in the motor for driving the vacuum pump so as to prevent any oil and gas escaping to the outside through the motor, in particular along the conductors feeding the motor stator coil. Because of the very high speed of rotation of the vacuum pump, e.g. about 6000 revolutions per minute (rpm), it is difficult to provide satisfactory sealing via the dynamic seals provided around the motor shaft between the motor and the gears. This results in a tendency for oil to escape outwards through the motor. In addition, the vacuum pumps used in industrial processes contain toxic and polluting gases which it is essential to prevent from escaping into the surrounding atmosphere.
In a known vacuum pump, as shown in FIG. 1, sealing is provided by a leakproof intermediate jacket between the rotor and the stator of the motor. FIG. 1 is a longitudinal section view showing the first end of the stator 1 of a vacuum pump 100 having a drive shaft 2 extending the pump rotor (not shown). The drive shaft 2 is engaged in a motor unit 200, where it is secured to the rotor 3 of the motor. The rotor 3 of the motor is mounted to rotate on bearings inside the stator 4 of the motor which includes a stator coil 11 fed by electrical conductors (not shown). The assembly comprising the stator 4 and the rotor 3 of the motor is inserted in a motor casing 5. Seals can provide sealing around the drive shaft 2 where it enters the motor casing 5 to isolate the atmosphere inside the motor casing 5 as much as possible from the upstream compartment 7 containing a set of gears 8 for coupling together the two parallel rotors of the vacuum pump 100. The gears 8 distribute rotary motion between the two rotors, with only one of the rotors being coupled in line with the drive shaft 2. The upstream compartment 7 containing the gears 8 also contains oil for lubricating the gears. In order to guide the rotor 3 of the motor radially, so as to reduce vibration, an additional bearing 15a is placed between the gears 8 and the rotor 3 of the motor.
The seals are not sufficiently effective to prevent lubricating oil for the gears 8 and toxic gases coming from the vacuum pump 100 passing into the outside atmosphere through the motor casing 200, and in particular along the conductors feeding the stator coil 11 of the motor. To prevent oil and gases migrating to the outside atmosphere through the motor unit 200, the known structure shown in FIG. 1 has a leakproof jacket 9, in the form of a coaxial bell, whose mouth 10 is received in leaktight manner around its entire periphery between two portions of the motor casing 5, i.e. a main portion 51 and a fixing base 52. The leakproof jacket 9 has a cylindrical intermediate portion 90 which is engaged in the airgap between the stator 4 of the motor and the rotor 3 of the motor, and which is connected both to the bell mouth 10 of the jacket and to an end wall 91 thereof.
A first drawback of such a known structure is its complexity due to the fact that it is necessary to provide and to assemble together a plurality of parts, comprising the leakproof jacket 9, and the two-part motor casing 51 and 52. That increases the cost of making the vacuum pump.
A second drawback is that the presence of the cylindrical intermediate portion 90 of the leakproof jacket 9 engaged in the airgap between the stator 4 and the rotor 3 of the motor means that it is necessary to have an airgap of relatively large width, thereby increasing the amount of electricity that needs to be consumed in order to drive the vacuum pump 100.
Another drawback is that the presence of the leakproof jacket 9 increases the length of the motor, moving the stator 4 and the rotor 3 thereof away from the vacuum pump 100, and increasing the cantilevered-out length of the drive shaft; that increases motor vibration, and also the noise generated by the assembly comprising the vacuum pump 100 and the motor unit 200, and it also requires the presence of the additional bearing 15a between the gears 8 and the rotor 3 of the motor.
Another drawback lies in the leakproof jacket, which is made of metal, being subjected to an alternating magnetic field in the airgap of the motor. This results in current being induced in the material constituting the leakproof jacket, in energy losses, and in additional heating of the motor. These losses increase with the frequency of the magnetic field and they become prohibitive in a four-pole motor powered at double frequency.
JP 07 317673 discloses a screw pump for various fluids. The drive motor is disposed in an intermediate zone of one of the rotor shafts, between the pump rotors and the gears coupling the shafts together. The motor casing is distinct from the casing for the coupling gears. That structure is neither designed nor adapted to resolve the problems specific to sealing vacuum pumps.
U.S. Pat. No. 6,002,185 discloses a motor for coupling to a valve in order to open it. The motor coil is embedded in the mass constituting the motor casing so as to avoid any cracking that could lead to corrosive moisture forming on the stator of the motor.
A particular object of the present invention is to avoid the drawbacks of known vacuum pump structures by proposing a new structure for a dual-rotor vacuum pump unit associated with a motor which is sealed in a manner that is simultaneously simple, low cost, and more effective.
The invention seeks to eliminate the leakproof jacket 9, replacing it by other means for effectively ensuring sealing and opposing any oil and gas migrating through the motor to the atmosphere.
To achieve these objects and others, the invention provides a dual-rotor vacuum pump unit comprising a pump stator with at least one axial internal cavity receiving two parallel pump rotors mounted to rotate on corresponding bearings and coupled together at a first end by a set of gears enclosed in a gear casing containing oil, the first end of one of the pump rotors being extended axially by a drive shaft engaged in the rotor of a motor unit for driving the vacuum pump, the motor unit having a stator coil and being enclosed in a motor casing extending the gear casing; according to the invention:
the motor casing and the gear casing form a common one-piece casing connected to the first end of the pump stator of the vacuum pump;
inside the common one-piece casing, the stator coil of the motor unit is embedded in a leakproof resin providing sealing that prevents oil and gas from escaping to the outside along the feeding conductors.
This avoids resorting to sealing means that are bulkier and more expensive and as a result production cost is reduced and reliability of the pump unit is improved.
This also avoids the losses that result from induction currents that are inevitably generated in the mass of material constituting a leakproof jacket engaged in the airgap between the rotor and the stator of the motor.
In an advantageous embodiment, the common one-piece casing includes an intermediate wall between a first compartment containing the motor and a second compartment containing the set of gears, and including a passage for the drive shaft with a dynamic seal to provide sealing around the drive shaft between the first compartment and the second compartment.
For convenience in assembly, the common one-piece casing may have an axial end opening closed in leakproof manner by a closure hatch.
Additional reduction in vibration is obtained by providing for the common one-piece casing to be connected to the first end of the pump stator via a bearing support having a first bearing for guiding the drive shaft located as close as possible to the motor. This reduces the cantilevered-out length of the drive shaft. Reduction in the total length and in the cantilevered-out length is further facilitated by the fact that impregnating the motor stator in the leakproof resin makes it possible to move it closer to its casing, since insulation distances can be reduced because of the dielectric quality of the leakproof resin.